S.T.
The answers here are all over the place. So here's another...
9 hours is a long time in the car with three little ones under 10. But are they 10, 8 and 6? or are they 1, 3 and 8? Big difference. The reason your husband is upset is becuase he feels that he can't control and protect the 4 people most precious to him. He worries about a flat tire or overheated engine and you on the side of the road with 3 kids. My husband wouldn't have been crazy about it either when the kids were little. Now that my kids are almost 14 and almost 17 we will frequently go on a 7-8 hour trip to visit my niece without my husband. But teens are different - they can help in an emergency, they won't wander onto the highway and they can help protect the famil.
I understand your desire to go see your folks and have a vacation. Is your car in good shape? Do you have AAA? Do you know anyone along the way that you can stop and visit? Do you have a plan for half way along the drive to let your kids stretch their legs and get their wiggles out?
Why don't you instead approach it like this - "hon I really want to go see my folks and bring the kids there. They are all dying to see eachother and it will give me a little break and breather. I know you worry about us and the drive. What can we do to plan the drive in a way that will make you comfortable? Ask his advice. Pay the $50 annually for AAA. Lay out detailed plans so he can know where you'll be along the way. Call him frquently from the car (the kids can be the ones calling). See if there's a way your parents can drive some of the way to meet you - so when you're at hour 7 or 8 you can meet up with them and your mom can join you in your car and your old child can go int he car with grandpa (or soemthing like that). Be creative in finding ways to calm your husband's worries and allow him to have input. defer to his judgement.
Ultimatley, he see his job as providing for and protectin his family - so help him figure out how he can do that on your 10 hour (each way) drive.
Good luck mama!